1. Field of the Invention
The invention relates to techniques for controlling the step response of electronic components, and with particular reference to electronic ignition systems in motor vehicles, particularly in the car sector.
2. Description of the Related Art
The tendency of using small-sized coils which considerably reduce both the occupied area and the costs of the system, also permitting the implementation of innovative solutions, such as the direct assembly on the engine head, without high-voltage wires, is continuously spreading.
The main disadvantage of using such systems is due to the fact that, since the primary inductance value is generally low driving by means of electronic components capable of switching high currents, e.g., IGBTs (acronym of Insulated Gate Bipolar Transistor), must be resorted to in order to provide sufficient energy during the spark generation phase.
An additional phenomenon found in these systems is the establishment of high parasite capacitance values, which may result in an underdamped step response.
In the described systems, the charging time of the ignition transformer (usually called xe2x80x9ccoilxe2x80x9d) is generally determined by a microprocessor which makes the IGBT conduct by means of a trigger pulse.
Working as a closed switch, the IGBT allows the flow of current in the primary winding of the coil. As soon as the current reaches a suitable value, the trigger signal switches to low and cuts off the IGBT, thus causing an overvoltage in the primary winding. The overvoltage, which is limited (typically to a value of approximately 400 V) by a zener diode in the IGBT, is transferred to the secondary winding via the secondary/primary turn ratio of the coil to generate the high voltage needed to produce the spark at the plug.
In practice, the IGBT is energized by a voltage step. In the described conditions (i.e., with an essentially inductive load), an overshoot of the collector voltage is generated when the IGBT exits the saturation region. This phenomenon is unacceptable because, returning to the secondary via the turn ratio, it can originate undesired sparks.
Implementation of the diagram shown in FIG. 1 has previously been proposed to overcome this problem. The diagram is representative per se of both the prior art and of an application according to the invention.
The IGBT is indicated by the corresponding acronym in the diagram in FIG. 1. The diagram also shows the respective collector C, gate G and emitter E terminals.
The collector-emitter line of the IGBT is interposed between the battery voltage B and the ground T in the ignition system connected in series to the primary winding P1 of the ignition transformer (currently called xe2x80x9ccoilxe2x80x9d). Reference P2 indicates the secondary winding of the illustrated transformer, which is structured to power a spark plug SP.
The control action described above is implemented by applying a step control signal generated by a control device such as, for example, a microprocessor MP, to the gate G of the IGBT via a resistor R.
The description above to this point corresponds to principles of operation and implementation criteria which are well known in the art and consequently do not require a detailed description herein.
Equally known is the aforementioned solution consisting in interposing a resistor with a suitable value between the collector C of the IGBT and its control terminal G, which resistor is indicated with numeral 1 in the diagram in FIG. 1.
Particularly, it is possible to see that as the value of the resistor 1 decreases, the detrimental phenomenon of overshoot described above gradually decreases to total disappearance.
This solution however clashes with another difficulty, related to the practical impossibility of operating in fully satisfying ways in solutions which, as in the case of electronic ignition systems, high voltage values, e.g., in the order of 400 V, occur on the primary winding P1 when the IGBT is switched off.
Particularly, in this application, the voltage on the collector C does not reach the desired value because the resistor 1 would cause the IGBT to be switched on again when it should be cut off.
The disclosed embodiments of the invention provide a solution that is capable of overcoming the described shortcomings.
Particularly, the solution according to the invention is based on the use of resistive elements with current saturated behavior correlated to voltage increase.
The phrase xe2x80x9ccurrent saturated behavior correlated to voltage increasexe2x80x9d herein indicates those resistive elements (usually made of semiconductor structures) susceptible of showing:
a normal resistive behavior in a first range of voltage values applied to the terminals, i.e., a dependency that is essentially linear of the intensity of the current through the component on the voltage applied across the terminals; and
a phenomenon of current saturation for which the value of the intensity of the current through the element remains approximately constant (i.e., increases only very slightly, according to a typical asymptotic pattern) with the voltage applied across the terminals of the element as the applied voltage exceeds the first range of values (i.e., exceeds a certain neighbourhood of threshold values).
Resistive elements of this type are known, as documented, for example in European Application EP-A-0 996 158.
An element of this kind is also described in a co-pending U.S. patent application entitled A Structure for a Semiconductor Resistive Element, Particularly for High Voltage Applications and Respective Manufacturing Process filed concurrently herewith in the name of the Applicant.